Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Molecular ion beams may be more easily transported at a higher energy and lower beam current than atomic ion beams. The atoms (including dopant species) in a molecular ion share an overall kinetic energy of the molecular ion according to their respective atomic masses. Furthermore, by having fewer ions implanted to obtain the same dose, any space charge effect in the ion beam and, consequently, beam “blow up” may be minimized.
Molecular ions, however, may be difficult to form and maintain during the ionization or implantation process. Use of an ion source, such as an indirectly heated cathode (IHC) or Bernas source, tends to break up molecules into atomic ions. For example, dopant molecules like BF3, PH3, PF5, AsH3, B2H4, or GeF5 tend to dissociate easily in the thermal plasmas formed using an IHC or Bernas source. Thus, atomic ions may be formed instead of molecular ions. A plasma doping system that generates a plasma of ions and neutrals and biases a workpiece for implantation may not adequately control electron temperature or other plasma conditions such as pressure, power, frequency, or gas flow to form molecular ions. Thus, molecular ions may dissociate within a high voltage sheath of such a plasma doping system. This may be due to the pressure inside the high voltage sheath. So, for example, BF3 may only survive as B+ ions after crossing the high voltage sheath and implanting a workpiece. Additionally, as the molecular ions increase in molecular weight, it may become more difficult to not dissociate these molecular ions into atomic ions or smaller molecular ions in a conventional ion source. Other ion source systems may have lifetime issues due to depositions on components of the ion source caused by molecular ion formation. Use of certain dopants may cause deposits of, for example, B, C, As, or P to build up on surfaces within the ion source. Accordingly, there is a need in the art for an improved method of forming molecular ions.